This invention relates to projection lenses and, in particular, to projection lenses for use in forming a large image of a small reflective object composed of pixels, such as, a reflective liquid crystal display (LCD), a digital mirror device (DMD), or the like.
Projection lens systems (also referred to herein as xe2x80x9cprojection systemsxe2x80x9d) are used to form an image of an object on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection). The projection lenses of the present invention are specifically tailored for use in very compact front projectors, where the projected image emerges from the projector and is sent onto an external wall or screen.
To achieve a high level of compactness, the illumination for such front projectors is preferably fed in from the side near the object end (short conjugate end) of the projection lens. In the case of DMDs, the pixelized panel is also offset in order to provide the appropriate illumination geometry and to allow the dark-field light to miss the entrance pupil of the lens. This dark-field light corresponds to the off position of the pixels of the DMD.
The basic structure of such a system is shown in FIG. 5, where 10 is a light source (e.g., a metal halide or a high pressure mercury vapor lamp), 12 is illumination optics which forms an image of the light source (the xe2x80x9coutputxe2x80x9d of the illumination system), 14 is the object which is to be projected (e.g., a Texas Instruments DMD of on and off pixels), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on a viewing screen (not shown).
As shown in FIG. 5, the illumination optics can include multiple lens elements 15, 16, 17, a light tunnel 18 (e.g., a light tunnel constructed in accordance with commonly-assigned U.S. Pat. No. 5,625,738), and a mirror 19 for folding the optical axis 20 of the illumination system and thus reduce the overall size of the projector. As also shown in this figure, the optical axis 20 of the illumination system intersects the optical axis 22 of projection lens 13 at an acute angle.
Projection lens systems in which the object is a pixelized panel are used in a variety of applications. Such systems preferably employ a single projection lens which forms an image of either a single panel which is used to produce red, green, and blue images or of three panels, one for red light, a second for green light, and a third for blue light. In either case, projection lenses used with such systems generally need to have a relatively long back focal length to accommodate the auxiliary optical systems, such as color wheels, beam splitters, etc., normally used with pixelized panels.
A particularly important application of projection lens systems employing pixelized panels is in the area of microdisplays, e.g., front projection systems which are used to display data. Recent breakthroughs in manufacturing technology has led to a rise in popularity of microdisplays employing digital light valve devices such as DMDs, reflective LCDs, and the like.
Projection displays based on these devices offer advantages of small size and light weight. As a result, a whole new class of ultra portable lightweight projectors operating in front-projection mode and employing digital light valves has appeared on the market.
To display images having a high information content, these devices must have a large number of pixels. Since the devices themselves are small, the individual pixels are small, a typical pixel size ranging from 14-17 xcexc for DMD displays to approximately 8 xcexc or even less for reflective LCDs. This means that the projection lenses used in these systems must have a very high level of correction of aberrations. Of particular importance is the correction of chromatic aberrations and distortion.
A high level of chromatic aberration correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field.
All of the aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, astigmatism, and distortion typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction, including correction of secondary lateral color, is thus needed from the projection lens.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset panels. In such a case, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
Low distortion and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of distortion or color aberration in the images of such interfaces.
The above-mentioned microdisplays and, in particular, microdisplays employing DMDs, typically require that the light beam from the illumination system is fed in from the side near the short conjugate side of the projection lens (see the discussion of FIG. 5 above). This leads to a requirement that the entrance pupil 24 of the projection lens 13 is located at or near the lens"" short conjugate side or, equivalently, that the lens"" aperture stop is located near the short conjugate side. Such a location for the aperture stop exacerbates the optical design problem. In particular, the nearly external entrance pupil means that there is almost no internal lens symmetry for facilitating the correction of xe2x80x9coddxe2x80x9d optical aberrations such as lateral color and coma. (Note that the nearly external entrance pupil can have the advantage of reducing heat buildup within the lens during operation.)
In addition to the foregoing, there is an ever increasing demand for greater compactness of projection lens systems. In terms of the projection lens, this translates into a requirement that the lens has a wide field of view in the direction of the image (screen). This requirement makes it even more difficult to correct the lateral color of the lens. Similarly, the requirement for a long back focal length also makes it more difficult to correct lateral color.
Achieving a short focal length (e.g., a focal length of around 30 millimeters), a long back focal length (e.g., a back focal length which is at least as long as the lens"" focal length), a wide field of view in the direction of the lens"" long conjugate (e.g., a field of view of at least 50xc2x0), and an aperture stop close to the short conjugate side of the lens, while still maintaining the high level of aberration correction needed for a projection lens system which employs pixelized panels is particularly challenging since these various requirements tend to work against one another. To do so while minimizing the number of lens elements used in the lens so as to control the cost and weight of the lens is even more demanding. As discussed and illustrated below, the present invention provides projection lenses which satisfy these conflicting criteria.
In view of the foregoing, there exists a need in the art for projection lenses for use with pixelized panels which have some and preferably all of the following properties:
(1) an aperture stop near to the short conjugate side of the lens;
(2) a high level of lateral color correction, including correction of secondary lateral color;
(3) low distortion;
(4) a large field of view in the direction of the image;
(5) a long back focal length (BFL);
(6) a short focal length; and
(7) a minimum number of lens elements.
To satisfy this need in the art, the invention provides projection lenses which have some and preferably all of the above seven features.
In particular, the invention provides a projection lens having a power "PHgr"0 and consisting in order from the lens"" long conjugate side to its short conjugate side of:
(A) a first lens unit (U1) having a power "PHgr"U1 and comprising at least one aspheric surface; and
(B) a second lens unit (U2) having a power "PHgr"U2 and consisting in order from the lens"" long conjugate side to its short conjugate side of:
(i) a first lens subunit (US1) having a power "PHgr"S1;
(ii) a second lens subunit (US2) having a power "PHgr"S2 and an overall meniscus shape which is concave towards the short conjugate; and
(iii) a third lens subunit (US3) having a power "PHgr"S3 and a surface S3 at its long conjugate side;
wherein:
(a) "PHgr"U1 less than 0;
(b) "PHgr"U2 greater than 0;
(c) "PHgr"S1 greater than 0;
(d) "PHgr"S2 less than 0;
(e) "PHgr"S3 greater than 0;
(f) BFLxe2x80xa2"PHgr"0xe2x89xa71.0; and
(g) the lens"" aperture stop is either within the third lens subunit or spaced from surface S3 in the direction of the long conjugate by a distance D which satisfies the relationship:
Dxe2x80xa2"PHgr"0xe2x89xa60.1.
Preferably, the BFLxe2x80xa2"PHgr"0 product is greater than or equal to 1.2 and the Dxe2x80xa2"PHgr"0 product is equal to or less than 0.05.
In addition to having the above BFLxe2x80xa2"PHgr"0 and Dxe2x80xa2"PHgr"0 products, the projection lenses of the invention preferably have a field of view xcex8 in the direction of the long conjugate of at least 50xc2x0 and preferably greater than 55xc2x0. Also, the projection lenses of the invention preferably have eight or less lens elements, e.g., seven lens elements, where doublets are treated as two lens elements. Considering doublets as a single component, the projection lenses of the invention preferably have six or less components, e.g., five components.